FMA Unit 1.4 Memory Organization

8051 Memory Organization

8051 memory divided in to two types

   A.  Program Memory (ROM)

·       It can access up to 64 KB program memory (60 KB external and 4 KB internal program memory).

·       When 4KB of Internal ROM (address from 0000H to 0FFFH ) as well as 60KB of External ROM (address from 1000H to FFFFH) have to be used then EA (bar) pin is connected to Vcc i.e. EA (bar) = 1

 ·       When whole 64KB of External ROM (address from 0000H to FFFFH) has to be used then EA (bar) pin is connected to GND i.e. EA (bar) = 0

   B.  Data Memory (RAM)

·       It can address upto 64 KB of external data memory and 256 bytes of internal memory.

·       The internal data memory is divided into THREE different blocks.

a.     Lower  128 bytes (00H to 7FH)

b.    Upper 128 bytes ( 80H to FFH)

c.     SFRs (Specials Function Registers)

Structure of Internal RAM (00H to 7FH)

It has 128 bytes Internal RAM which organized in three areas:

A.1 -  Register bank area

·       There are four register banks ie. 0 to 3 each of 8 byte

·       Each bank made up of eight registers named R0 to R7.

·       In total Working registers from address 00H to 1FH organised as four register banks.

·       If any of the banks is not selected, then programmer can be use the area from 00H to 1FH .

Upon power ON or RESET Bank 0 is selected by default.

A.2 - Bit Addressable area of 16 Bytes

·       The microcontroller has reserved of internal RAM whose address ranges from 20H to 2FH.

·       These 16 Bytes provide us with 16 × 8 = 128 bits forming address bit.

·       The microcontroller has given addresses to these bits ranging from 00H to 7FH.

·       These location are called as Bit Addressable Locations.

A.3 -  General Purpose RAM area

·       This RAM is also called as scratch pad area

·       It lies above bit address area and as address 30H to 7FH

·       This RAM can be used as data RAM

Use of Internal RAM

·         It is not essential to utilise the entire area of internal RAM.

·         The microcontroller has instructions that allow the internal memory location to be use as data pointer.

·         The registers R0 & R1 in the four register banks can be used as a pointer to point the internal and external RAM both.

·         When the register R0 & R1 are used to access the external RAM, the 8 bits will be consider as LSB of 16 bit address and MSB is taken to be 00H by default

External Memory

·       External memory is used when the internal ROM and RAM memory is not sufficient on chip.

·       Two separate external memories are available by different control pins for enabling external ROM and RAM chips.

·       In 8051 family, external RAM of upto 64 KB and external ROM of upto 64 KB can be added on any chip.

Special Function Registers (SFRs)

·       SFRs are placed in the address space immediately above 128 bytes of RAM from 80H to FFH.

·       Out of these 128 Memory Locations (80H to FFH), there are only 21 locations that are actually assigned to SFRs. Each SFR has one Byte Address and also a unique name which specifies its purpose.

·       Since the SFRs are a part of the Internal RAM Structure, you can access SFRs as if you access the Internal RAM. The main difference is the address space: first 128 Bytes (00H to 7FH) is for regular Internal RAM and next 128 Bytes (80H to FFH) is for SFRs.

·       The SFRs are used to program and control different hardware peripherals like Timers, Serial Port, I/O Ports etc

·       Suppose some of the address (80H to FFH) is not used, when writing the data to such unused location the data will be lost it not stored anywhere.